I think what you have is the perfect amount of complexity, and easily extendable. I use callbacks like this all the time in my code, but also my users are part of my team at work so they don't have far for help when the callback API isnt clear :P most of my users dont even know callbacks are a thing.
If your list of supported events gets large or you want to cheaply support future changes to the server, you could pass all event names and data through a handler callback and make users handle multiple event types.
One of the things that makes plain callbacks nice is it's an easy way to let your users decide how they want to integrate with your library. If your callbacks are called from a background thread, it's up to the user to figure out how they want to pass the event to other threads. Async users can use their runtime of choice to submit tasks from a callback.
You're right, I'm being judgemental about the English stuff... I think Im just especially suspicious of software that is written by people who clearly have the skills to pwn my machine when the software has access to ring0 and is used to boot and install entire oses. It's a malware gold mine. Even if the project is completely on the level, it's a high value target for adding malware because of the level of control you get over a machine (just like grub or syslinux of course, I'm mainly thinking about iventoy for that point). Plus as an American I'm definitely automatically more suspicious of software from China :/ not great but it's true.